Access networks are presently experiencing rapid growth around the world. Both residential and business customers are demanding increasingly higher bandwidths from their Internet service providers who in turn are pressed to implement networks capable of delivering bandwidths in excess of 100 Mb/s per customer. For this application, passive-optical-networks (PON) are particularly well suited as they feature lowest capital-equipment expenditures relative to point-to-point and active optical networks. The book by C. F. Lam, Passive Optical Networks: Principles and Practice, Academic Press, 2007, and publication by C-H. Lee, W. V. Sorin, and B. Y. Kim, “Fiber to the Home Using a PON Infrastructure”, IEEE J. Lightw. Technol., vol. 24, no. 12, pp. 4568-4583, 2006 give good introduction into this technology. Wavelength-division multiplexing in passive optical networks (WDM-PON) is one of the actively investigated as next-generation optical network architecture. WDM-PON provides higher bandwidth per user than any other network architecture and hence potentially offers the lowest cost per unit of bandwidth to the user. However, the key difficulty in such a system has been the cost of the components, particularly arising from the need to transmit light at one wavelength for a specific channel and also receive information at any one of several other wavelengths at the user end in the so-called optical network unit (ONU). WDM optical and optoelectronic components traditionally exhibit high cost, among other issues, due to precise wavelength definitions in such systems. A dramatic cost reduction is achieved by eliminating wavelength-specific transceivers at the ONU in the colorless WDM-PON system, also referred to as a system with wavelength-agnostic transceivers in the ONU.
In a colorless optical network, the wavelengths emitted and received by the transceiver in the ONU are defined in the remote node or the central office rather than in the transceiver at the ONU as is well known in the art—see book by Lam cited above. Further reduction in complexity and cost is realized by using self-seeding or self-injection locking technique, in some cases combining a simple mirror or a Faraday mirror with self-seeded structures. FIG. 1 illustrates an example of prior art in this field. A self-seeded WDM-PON network system 100 comprises of central office 101 (CO), remote node 106 (RN), and a multiplicity of optical network units (ONU) of which only one 104 is shown connected to the remote node 106 using distribution fibers of which only one is shown 103. The central office 101 is connected to the remote node 106 with trunk fiber 111, while the remote node 106 is connected to each of the ONUs using a distribution fiber 103. The distribution fiber 103 has length LD that typically ranges from 500 m to 2 km. The trunk fiber 111 has length LT that may be 10 km or more, depending on the application. Other ONUs, not shown in FIG. 1, connect to the RN 106 using distribution fibers of various lengths to the available ports 108 on the RN 106. The CO 101 sends independent information or a broadcast to each of the ONUs 104 over the trunk fiber 111 by encoding each data stream into an optical signal of a specific wavelength λDk, where subscript D refers to downstream data traffic, i.e. traveling from the CO 101 to the ONUs 104, and the subscript k indicates the k-th wavelength of a set of wavelengths. Such an optical signal is referred to as wavelength division multiplexed (WDM). The number of optical signals and associated wavelengths of these signals are predetermined by the system hardware and optical standard (eg. ITU). The downstream wavelengths λDk are equidistantly separated and form the downstream WDM band denoted with ΣλD. On the ONU side, each ONU 104 sends a data stream towards the CO 101 at a specific wavelength associated to that ONU 104. The so-called upstream wavelengths λUk are different from the downstream wavelengths and form the upstream WDM band ΣλU.
The ONU 104 is a transceiver for digital data. The transmitted data is upstream data, and received data is downstream data. The transmitting chain of the ONU 104 comprises a gain and modulation chip 105 (GMC), which may be a reflective semiconductor optical amplifier (RSOA) or a Fabry-Pérot laser with a low-reflectivity coating on its front facet from which the light is coupled into a segment of a distribution fiber 103 via a diplexer 110 and optionally a Faraday rotator 109. The diplexer 110 separates the upstream optical signal, denoted with wavelength λUk, from the downstream optical signal denoted with wavelength λDk. The other end of the GMC 105 is terminated with a high-reflectivity mirror 107. A certain amount of optical power is transmitted through said high-reflectivity mirror and captured by a monitor photo diode (not shown in FIG. 1) whose function is to enable monitoring of the optical power emitted from the GMC 105. The ONU 104 is used to convert upstream digital data entering the ONU 104 via electrical terminals 121 to optical signal emitted along the distribution fiber 103. The electrical data entering through the digital interface 121 is processed by the physical-layer chip 120, and is used to intensity modulate the GMC 105 via electrical interconnect 122 thereby encoding the data into the upstream optical signal λUk. The downstream optical signal arriving along the distribution fiber 103 is redirected to a receiver 124 (comprising a photodetector and a transimpedance amplifier) by the diplexer 110. The digital data is received by the optical receiver 124 and is electrically connected using lines 123 to be processed by physical layer chip 120 and delivered to the network processor (not shown) via the electrical terminals 121. The diplexer separates the upstream from the downstream wavelengths as is known in the art.
The remote node 106 comprises an array-waveguide grating 116 (AWG) which serves as a wavelength-division multiplexer/demultiplexer (WDM MUX/DEMUX) for both the downstream and upstream signals. A downstream WDM optical signal arriving at the common port 112 of the AWG 116 is demultiplexed into a number of distinct wavelengths and redirected to the distribution ports 108. The downstream optical signal frequencies are equally distributed within the downstream optical band. The upstream optical signals generated by the multiplicity of ONUs 104 arriving each at a different distribution port (distribution ports 108) and each having a different wavelength λUk is multiplexed into a single output at the common port 112 of the AWG 116. The optical signal containing multiple wavelengths equally distributed within the upstream band ΣλUk is emitted from the common port 112. The AWG 116 is typically a cyclical array waveguide grating which provides at least two bands with multiple equally-spaced wavelengths in each band as is illustrated in FIG. 2.
FIG. 2 illustrates qualitatively the upstream emission spectrum 252 and the downstream spectrum 253 as it is defined by the cyclic arrayed-waveguide grating (AWG) 116 (shown in FIG. 1). PSD stands for power spectral density. The upstream band ΣλU 262 comprises a multiplicity of equally spaced wavelengths λUk 266, where k=1, 2, . . . , N, while the downstream band ΣλD 263 comprises a multiplicity of equally spaced wavelengths λDk 265, where k=1, 2, . . . , N. Not all wavelengths are used at any given time in a network, but the wavelengths and the band are nevertheless allocated for optical signals. The upstream and downstream bands are separated in frequency: The upstream band may use wavelengths in the ITU C-band, while the downstream signal may use wavelengths in the ITU L-band. The AWG may be adjusted so that the upstream and downstream band separation equals the AWG cycle 261 as is shown in FIG. 2.
Common port 112 of the AWG 116 is connected to the trunk fiber 111 via an optical coupler 118 (at least three ports). One port of the optical coupler 118 takes apportion of the upstream optical signal, passes it through a 45-degree Faraday Rotator 113 to be reflected on a fiber mirror 114 as shown in FIG. 1. The sequence Faraday Rotator 113 and the fiber mirror 114 is commonly referred to as a Faraday Rotating Mirror (FRM) and comprises a self-seeding component that returns a portion of the optical signal emitted from port 112 back into the same port 112. Under normal operation, a portion of the optical power of the upstream optical signal ΣλU emitted from the common port 112 is reflected back using the FRM 113/114 to the ONUs 104 for seeding the GMC 105, while a portion of the optical signal with information encoded in its modulation is transmitted through the optical coupler 118 and delivered to the central office 101 via the trunk fiber 111.
The principle of operation of the network system 100 is as follows: The GMC 105 emits spontaneous emission in a broad wavelength band which includes the upstream wavelengths covered by the AWG upstream band 262. As only one port (distribution ports 108) is connected to any one GMC, this broad upstream band is spectrally sliced by the AWG 116 and only a narrow wavelength range around a specific wavelength λUk is passed through to the optical coupler 118, reflected by the FRM 113/114, and then sent back into the same GMC 105. The seeding signal is now amplified and directly modulated by the GMC 105: Data is encoded into the modulation of the gain and hence a modulated signal is now emitted towards the remote node 106 via the distribution fiber 103. This signal is again spectrum-sliced by the AWG 116 and partially transmitted by the optical coupler 118 to carry the information to the central office 101. A portion of the optical power is returned to the GMC 105 for further seeding. The optical feedback realized by the self-seeding using FRM 113/114 enables stimulated emission and gain clamping within the GMC 105. The emission wavelength is defined by the passband of the AWG: It is the port number (one of the AWG ports 108) that defines the wavelength at which this seeding and laser oscillation happens. In this way, each of the ONUs provides different upstream wavelength. The central office 101 may have analogous optical transmitters operating in the downstream band (λDk) to deliver information to the ONU.
The GMC 105 is typically polarization sensitive: it provides higher gain for only one polarization of the incident seeding light, but may be designed to be polarization insensitive. Polarization insensitive components generally provide less optical gain than the kind that is polarization sensitive. The distribution fiber 103, the AWG, and other optical components the optical signal traverses while making its trip from the GMC 105 and the seeding component 113/114 exhibit a degree of time varying birefringence inherent in the fiber and other waveguide components. For this reason, the optical beam returning to the GMC for seeding will generally experience unknown and fluctuating polarization. This polarization fluctuation and mismatch very often produce an unstable link, a problem that is to a certain degree resolved by using two 45-degree Faraday rotators in the link: one within the remote node 106 FRM 113 and another FR 109 in front of the GMC 105 within the ONU 104. By introducing an FRM in the remote node 106, any retardation in the polarized beam emitted from the GMC 105 on its way to the FRM 113/114 in the remote node 106 is compensated during the beam's travel from the FRM 113/114 back to the GMC 105. Namely, the FRM 113/114 exchanges the electrical field components along the ordinary and the extra-ordinary axes and the returning optical signal undergoes approximately equal change in polarization with the exception that the opposite electric field axes are affected. The resulting beam reaching the GMC 105 is linearly polarized with polarization orthogonal to the polarization of the emitted beam. Inserting another Faraday rotator 109 rotates the polarization of the returning beam to match the original emitted signal polarization. Experimental evidence shows that this improves the stability of the system compared to architectures that employ only a mirror without the rotating element in the seeding element.
An array-waveguide grating (AWG) is a passive optical component that is ubiquitous in optical networking used for filtering, separating, combining, and routing signals of different wavelengths as is well known in the art. Its use and principle of operation are described in publicly available texts, such as, “WDM Technologies: Passive Optical Components” by A. K. Dutta, N. K. Dutta, and M. Fujiwara, published by Academic Press in 2003. The AWG is most commonly implemented using planar lightwave circuit technology. Planar lightwave circuit technology (PLC) is described in many publicly available texts such as “Planar Circuits for Microwaves and Lightwaves” by Takanori Okoshi, published by Springer Verlag 1985 and in “Frontiers in Planar Lightwave Circuit Technology: Design, Simulation, and Fabrication” published by NATO Science Series, 2006. The physical properties of the dielectrics used in building the AWG are temperature dependent and consequently this temperature sensitivity results in a shift of the filter wavelengths. It is well known in the art today that this temperature variation can be efficiently compensated by using so-called athermal array-waveguide gratings.
There has been a number of proposed implementations of the self-seeding, often referred to self-locking, technique in the industry.
All self-seeding architectures are vulnerable not only to polarization stability issues but to modulation stabilization issues. The seeding optical signal returning to the ONU 104 from the FRM 113/114 is modulated. The modulation is uncorrelated to the data stream that needs to be transmitted from the GMC 105. The presence of this residual modulation in the seeding light deteriorates the optical signal emitted from the ONU because now the emitted signal contains both the new amplitude modulation and the residual modulation. There are several approaches explored to reduce the effect of this residual seeding light modulation: the most common approach is to drive the GMC into saturation where differential gain is lower, there are reports of using electronic compensation of the modulated signal, and finally performing optical modulation averaging of the seeded light within the remote node. The latter approach stabilizes the optical link by removing the modulation in the seeded light stream The network architectures disclosed in prior art deliver very high bandwidth per client, but they have not yet been widely deployed due to a number of yet unresolved issues some of which include low seeding stability, low link margin, and high cost. Therefore, an unmet need for a low-cost high-performance WDM-PON solution exists in the industry. This application discloses optimal coupler-based modulation-averaging reflector structures for low-cost implementation for self-seeded colorless optical networks with improved stability and link margin.